Integrated high-gain amplifiers are often used to amplify signals having relatively small amplitudes, i.e., having relatively low power. For example, such amplifiers are typically used to amplify signals received via optical fibers.
Before shipping such an amplifier, the manufacturer typically measures its bandwidth and, if necessary, adjusts the bandwidth to within the customer's specification. Because the customer will typically use the amplifier in a precision application, the bandwidth of the amplifier typically must be within a relatively tight frequency range. Unfortunately, variations in the amplifier's components caused by variations in the semiconductor process used to manufacture the amplifier may cause the amplifier's bandwidth to fall outside of this range. Furthermore, different customers may want different bandwidths. Consequently, to increase the process yield and to decrease the number of different amplifier versions that are manufactured, the manufacturer often designs these amplifiers to have adjustable bandwidths. During testing of an amplifier, the manufacture checks and adjusts the amplifier's bandwidth as discussed above.
Unfortunately, as discussed below in conjunction with FIG. 1, the techniques that manufacturers use to measure and adjust the bandwidths of integrated high-gain amplifiers may be unable to set the bandwidths with the precision that some customers desire.
FIG. 1 includes a block diagram of a conventional integrated circuit (IC) 10, which includes a differential, high-gain, bandwidth-adjustable amplifier 12 for amplifying a phase-modulated input signal, and includes a block diagram of a conventional setup 14 for checking and adjusting the bandwidth of the amplifier 12. In addition to the amplifier 12, the IC 10 includes a bandwidth-adjust terminal 16, differential input terminals 18 and 20, and differential output terminals 22 and 24. The amplifier 12 includes a filter 26 that has an adjustable bandwidth and that is coupled to the terminals 16, 18, and 20, one or more gain stages 28 coupled to the output nodes of the filter, and an amplitude-limiting output stage 30 coupled to the output nodes of the stages 28 and to the output terminals 22 and 24. In normal operation, because the input signal is phase modulated, there is no information contained in the signal amplitude; consequently, the output stage's “clipping” of the signal destroys no information. But as discussed below, this clipping can cause inaccuracies during the bandwidth measuring and/or adjusting of the amplifier 12. Furthermore, the setup 14 includes an amplitude extractor 32, a bandwidth adjuster 34 such as a personal computer (PC), and a sine-wave generator 36.
Unfortunately, the clipping action of the output stage 30 may cause the set up 14 to set the amplifier's bandwidth outside of the specified range. To set the bandwidth of the amplifier 12, the bandwidth adjuster 34 first causes the generator 36 to generate a sine wave having a frequency, for example 100 MHz, within the pass band of the filter 26. Then, the adjuster 34 stores the peak amplitude of the output pass-band sine wave across the terminals 22 and 24 as provided by the extractor 32. Next, the adjuster 34 causes the generator 36 to generate a sine wave having the cutoff frequency desired for the amplifier 12, for example 1 GHz. Then, the adjuster 34 adjusts the bandwidth of the filter 26—the bandwidth of the filter is the bandwidth of the entire amplifier 12 provided that the stages 28 and 30 each have significantly higher bandwidths than the filter—until the peak amplitude of the output corner-frequency sine wave across the terminals 22 and 24 is 3 dB down from the stored peak amplitude of the output pass-band sine wave. But if the output stage 30 clips the output pass-band or corner-frequency sine waves, then the peak amplitude of the clipped sine wave is nonlinear. Consequently, the error introduced by this nonlinear amplitude may cause the adjuster 34 to set the bandwidth of the amplifier 12 outside of the specified range.
To prevent the occurrence of such a clipping-induced error, the generator 36 typically sets the peak amplitudes of the input (across the terminals 18 and 20) pass-band and corner-frequency sine waves small enough so that the corresponding output (across the terminals 22 and 24) sine waves are not clipped.
But unfortunately, reducing the peak amplitudes of the input sine waves may also cause the set up 14 to set the amplifier's bandwidth outside of the specified range. Reducing the peak amplitudes of the input sine waves typically decreases the signal-to-noise (S/N) ratios of the input and output sine waves, and thus effectively increases the noise levels on these signals. This effective increase in the noise levels makes it more difficult for the amplitude extractor 32 to determine the peak amplitudes of the output sine waves, and thus may cause the adjuster 34 to set the bandwidth of the amplifier 12 outside of the specified range.
And although a high-precision setup 14 may be able to accurately set the bandwidth of the amplifier 12 using reduced-amplitude sine waves, such a setup is typically expensive and may take a relatively long time to adjust the amplifier's bandwidth because of the increased noise levels on the output sine waves.